Signal processing devices typically utilize an input circuit to interface between a first circuit or device and a processor. A common format of an input signal is known as a differential signal. A differential signal has its information content expressed by a difference either in phase or magnitude of two signals.
For example, in one form a differential signal pair may be two square wave signals that differ one hundred eighty degrees in phase and which transition between zero and two volts. In this example the common mode voltage of the differential signal pair is one volt.
As various consumer wireless products, such as pagers, telephones, etc. developed there has been inconsistency in the industry as to what voltages were used for the information signals. In particular, differential signal inputs for existing wireless products typically have a wide variety of voltages. For example some products transmit a differential signal having a low common mode voltage such as 0.2 volt. Other products transmit a differential signal having a significant higher common mode voltage value such as 1.5 volts. As a result of the potential of receiving a wide range of input signal voltages, the input transistors must be physically sized to be large enough to receive the largest expected voltage and not be damaged. However, the large sizing of the input transistors results in slower switching speeds for those transistors. Also, existing common mode input circuits usually exhibit the characteristic that if their inputs receive differing common mode voltages, the output voltage that such input circuits provide will also vary. The resulting variation of the output voltage limits the range of the output signal of the input signal because the minimum voltage of the output signal is proportional to the common voltage input. A limitation in the range of the output signal results in lost resolution and accuracy.
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